Cooling is critical to the success of concentrator systems that focus light onto photovoltaic cell(s), otherwise known as Concentrated Photovoltaic (CPV). Cell cooling in these systems may be either passive or active. Most CPV systems currently rely on passive cooling. While passive cooling has obvious advantages, such as simplicity and reliability, it has the significant disadvantages of being material intensive and therefore costly as well as uncontrollable, and less effective than a typical active cooling system. Passive cooling heat sinks are typically Aluminum with high surface area that is required for natural convection heat transfer; this mass of Aluminum is a significant cost component for CPV systems. Further cooling elements usually only add weight to a system but do not significant increase the structural strength, if at all. Such compact passive heat sinks with high surface area benefit very little from radiation heat transfer to the environment. In addition, because CPV systems use passive cooling to dissipate the heat from the PV cells to the atmosphere, there is little or no possibility of collecting the heat for beneficial uses. Such as can be done with emerging Concentrated Photovoltaic & Thermal (CPVT), essentially a subclass of CPV, whereby the Photovoltaic (PV) cell is actively cooled and heat is captured in a liquid heat transfer medium for beneficial use or to be dissipated elsewhere.
Typical parabolic trough concentrated PV system use a single reflection to focus light onto the PV cells. This has the advantage of not incurring losses from a second or successive reflection. However, it has the disadvantage that during the tracking of the sunlight either some light must be lost or the PV cell will have some dark areas, or a combination of both. Additionally, with the dispersion of the concentrated light due to solar ray angle, imperfect specular reflection, as well as various tolerances, limit the maximum concentration. To avoid this problem it is necessary to reflect at least a portion of the light a second time.
In flat panel PV modules spacing between the PV cells is not of critical importance, thusly spacing of several millimeters is acceptable. However, in CPV systems, where great care and cost have been expended to collect the light to a narrow linear focus, cell spacing is important. The traditional methods of tabbing solar cells to form the module into a string technically works, but with the cost of lost concentrated light and thereby efficiency. A fraction of a millimeter, the minimum to provide electrical isolation, is optimal for a parabolic trough CPV cell array.
In the state of the art parabolic troughs and other such linear concentrators for CPV, the PV cell buss bars are exposed to the concentrated light. This directly reduces the efficiency by the proportion of area covered by the buss bar since the light is reflected away from the cell and/or converted into heat. To reduce or eliminate this loss the buss bars should be removed or protected from the concentrated light. In concentrator cells, the proportion of buss bar coverage to active cell area can be high, even 20% or more. A typical technique for this has been to use rear surface contacts only. However, this is a costly and largely unnecessary approach for many applications.
In the state of the art parabolic concentrators for both CPV and concentrated thermal applications, little or no attention has been paid to controlling, reducing, and/or minimizing the resultant forces of lift and torque due to high speed winds, i.e. in excess of 90 mph. In high speed wind conditions, a parabola may have very high lift, several thousands of pounds force depending on the trough size. In conjunction with lift, they may also develop high torque, sufficient to damage the structure or break the constraints holding it from turning. If aerodynamically unmodified troughs are placed on rooftops, significant damage may occur to the structure do to high speed winds.
In all environments, mirrored surfaces of parabolic troughs require cleaning at various periodicities. This is usually accomplished by spraying the mirrors with water from a vehicle with a water tank and spray apparatus, which drives by the parabola.
This invention is warranted by the shortcomings of other parabolic trough concentrators. Specifically, the following areas are poorly or have entirely not been addressed: aerodynamic issues; material intensity, which contributes to high cost; concentrated light utilization, only converting a minor portion of concentrated light to electricity for beneficial use and then throwing away the heat as opposed to collecting it for beneficial use also; using passive cooling, those few systems which use active cooling use centralized heat exchangers as opposed to each trough having its own built in radiator which is much more efficient in terms of parasitic cooling loads and is more cost effective than centralized cooling.